1. Field of the Invention
This invention relates to a focus detecting apparatus in an auto focus camera.
2. Related Background Art
A charge accumulation type light receiving element, for example, a CCD image sensor, is used in a focus detecting apparatus in a camera. Where such a kind of sensor is used, the illumination of the element surface varies from a high luminance to a low luminance depending on an object to be photographed and therefore, the accumulation time is varied and controlled so that the signal level to be processed becomes substantially constant. There have been the following methods of controlling the accumulation time. One of them is a method as disclosed, for example, in U.S. Pat. No. 4,660,955 wherein the luminance of an object is monitored by a light receiving device differing from an image sensor and accumulation is terminated when the monitoring output reaches a predetermined level. Besides this method, there is also a method wherein charges are accumulated in advance in an image sensor for a certain time and after the termination of the accumulation, a signal from the image sensor itself is introduced and the next accumulation time is determined in conformity with the signal level.
The prior arts as described above have suffered from the following problems. In the former, the image sensor is a CCD or the like and therefore, it is difficult to individually detect a monitoring output corresponding to each light receiving element of the image sensor. As a result, it is unavoidable to monitor the average value of received light in a certain degree of range. If so, when as shown, for example, in FIG. 8 of the accompanying drawings, there is an object of high luminance in the fashion of a spot, if monitoring is effected at the average value, there has been the problem that the peak value exceeds the dynamic range of signal processing and accurate focus detection calculation is impossible. Also, in the latter method, accurate focus detection calculation is possible, but such an accumulation time that the sensor output assumes a proper level is obtained from the last accumulation time and the signal level thereof and therefore, there is no accumulation time data in the first accumulation time after the closing of the power source switch, and this has led to the disadvantage that if accumulation is effected with the initial value set in a certain accumulation time, several accumulations must be effected until the sensor output becomes proper and thus, much time is required for convergence.